So I've waited, and I've seen, and I continue to wait and see, but I still don't see how this is going to end any other way that with the epitaph for Angel being "he did the wrong thing for the right reasons"--that'll be the final nail (stake?) in the coffin of the character, the theme and point given to the audience.
A couple of theories have been put forth though, that some of the more nasty things that Twilight has done may have legitimate justification--Amy and Warren could be just self-sabotage (the guy who builds a batcave with the government funding to anihilate a supernatural terrorist organization should not be in charge of anything, and Angel would know that), that Angel's reasoning rather than 'hey lets hang around with some crazy rapists', the idea was to put really incompetant screwups in really important places. Which I can see why he'd recrute them, but still...he's killed people. He set up people to die, pointed scary rival forces directly at Buffy...With the whole "evil british Slayer" thing, rereading that hints heavily at the new developments in retrospect--there's all this talk "Queen" of Slayers, one that may be there to end them all, evil Slayer seems stronger than other Slayers. Later Angel states that "this was happening," and he was 'pushing' Buffy towards the Superpower cocktail. Maybe his inviting so much danger and manipulating things like a bastard was needed to make sure Buffy got this power, not just cause 'Yay Buffy gets superpowers' but specifically because SOMEONE was going to get this ability and he needed to make certain it was somebody who could handle it, unlike, Guinevere the psychotic Slayer?
As for what the power means, it could go a lot of different ways. Buffy and Angel's ability resonates with one another, and Giles obviously knows something about it. Something which seems to be cosmically good or bad, but bad for the girl in question. I'm guessing it's like some duality thing. We know that the Slayer powers are demonic in nature and they come from 'the essence of the demon,' I've long since suspected that the Old One that they used to get that essence was whatever one mixed blood with humans to create the first vampire anyhow. Now there's "Ultimate Slayer" and "Ultimate Vampire" and it's going to come down to them having a giant battle (or f^%*&^ing). It's definately going to be a duality thing, avatars or either rivaling, or forces of the same 'unholy power'--maybe it's a huge prophecy about ending all magic(that being a destined event, the end of the Slayer Line as there is no more need, world is safe from the forces of darkness forever) maybe that line is Angel's loophole to get rid of the precariously-placed world of the Buffyverse, maybe it's just that these two superbeings will end up summoning back this original Old One(or Ones--if my above theory is correct about the source of the Slayer powers, maybe this is the final price of the spell that created the Slayer, and the Slayer line turns into the worst monsters of them all--or lets the Old Ones return). But it'll probably be a real confrontation (one that Angel seemed intent on avoiding, eager to take a third option). Slayer wins, yay? Maybe. Vampire wins, destruction forever. - -wait a minute, wasn't there some prophecy about Angel, the apocalypse, him causing the ruination of all that is good?
...A prophecy which I fully expect to be ignored. I thought maybe they might make a point about Shanshu, but now I take that back--it's immaterial to this, and they'll try to sweep it under the rug. If it shows up as part of Angel's reasoning I'll be very pleased, but surprised. Some people say that because he's out in the sunlight and not bursting into flames, he must be human again and already shanshu'd, but I disagree--there's no shanshu anymore. It's not important (plus, invulnerable-all-over and all that). I feel, for all intents and purposes, Angel 'never went to LA', and became a hero in his own right--they've already just glossed over it, and now he's just to be Buffy's vampire boyfriend again.